1. Field of the Invention
The invention relates to an exhaust gas control apparatus for an internal combustion engine, in which a NOx storage reduction catalyst is recovered from poisoning caused by a sulfur component.
2. Description of the Related Art
A NOx storage reduction catalyst (hereinafter, the NOx storage reduction catalyst may be referred to as “NOx catalyst”) is provided in an exhaust passage. The NOx catalyst may be poisoned with sulfur oxide (SOx) (i.e., sulfur poisoning may occur). In this case, it is necessary to recover the NOx catalyst from sulfur poisoning so that a catalyst control function is restored. Therefore, a sulfur poisoning recovery process is performed at given time intervals. In the sulfur poisoning recovery process, a temperature of the NOx catalyst is increased to a temperature range in which SOx is released (hereinafter, referred to as “SOx release temperature range”), and the NOx catalyst is brought into a reducing atmosphere. For example, Japanese Patent Application Publication No. JP (A) 2000-045753 discloses an exhaust gas control apparatus in which an accumulated amount of SOx in a NOx catalyst is calculated based on a concentration of SOx that is detected by a SOx sensor provided at a portion downstream of the NOx catalyst; and when the accumulated amount exceeds a predetermined limit, a temperature of the NOx catalyst is increased to a SOx release temperature range (i.e., the temperature range of approximately 600° C. or higher), and an air-fuel ratio of exhaust gas (hereinafter, referred to as “exhaust gas air-fuel ratio”) is made rich for a predetermined time period so that the NOx catalyst is recovered from sulfur poisoning.
Also, in the sulfur poisoning recovery process, SOx released from the NOx catalyst reacts with hydrocarbon (HC) and carbon monoxide (CO), and thus hydrogen sulfide (H2S) is generated. Since H2S causes sulfur odor, an amount of released H2S needs to be reduced. Accordingly, Japanese Patent Application Publication No. JP (A) 2003-035132 discloses an exhaust gas control apparatus in which a sensor for detecting a concentration of H2S is disposed at a portion downstream of the NOx storage reduction catalyst, and the sulfur poisoning recovery process is controlled based on an output value of the sensor.
However, in the exhaust gas control apparatus using the SOx sensor disclosed in the Japanese Patent Application Publication No. JP (A) 2000-045753, since the amount of released SOx is not detected during the sulfur poisoning recovery process, the sulfur poisoning recovery process may be continued even when almost no SOx is released from the NOx catalyst, or the sulfur poisoning recovery process may be excessively performed and the amount of released H2S may not be reduced. In the exhaust gas control apparatus using the H2S sensor disclosed in the Japanese Patent Application Publication No. JP (A) 2003-035132, though generation of H2S is suppressed, the actual amount of released SOx cannot be detected during the sulfur poisoning recover process, as in the exhaust gas control apparatus disclosed in the Japanese Patent Application Publication No. JP (A) 2000-045753. Therefore, the sulfur poisoning recovery process may be continued while SOx is not sufficiently released. That is, a condition on which SOx is released from the NOx catalyst does not necessarily match a condition on which H2S is generated. Accordingly, when the sulfur poisoning recovery process is performed based on only the concentration of H2S, the sulfur poisoning recovery process may not appropriately proceed.